Coating finishes, particularly exterior coating finishes in the automotive industry, generally are applied in two or more distinct layers. Often, one or more layers of primer coating composition is applied to the uncoated substrate first, followed by one or more topcoat layers. Each of the layers supplies important properties toward the durability and appearance of the composite coating. The primer coating layers may serve a number of purposes. First, the primer coating may be applied in order to promote adhesion between the substrate and the coating. Secondly, the primer coating may be applied in order to improve physical properties of the coating, such as corrosion resistance or impact strength (e.g., for improving resistance to gravel chipping). Third, the primer coating may be applied in order to improve the appearance of the coating by providing a smooth layer upon which the topcoat layers may be applied.
In the process of finishing the exterior of automobiles today, metal substrates are usually first coated with an electrocoat primer. While the electrocoat primer provides excellent surface adhesion and corrosion protection, it is often desirable to apply a second primer layer. The second primer layer may enhance the corrosion protection of the finish, but it also serves to provide a smoother surface than the electrocoat primer. This second primer layer prevents the orange peel of electrocoat surface from telegraphing through to cause the surface of the final topcoat layer to be less than the desired smoothness. The second primer also serves to provide a barrier layer between the electrocoat primer layer, which usually contains aromatic moieties and other materials that can cause yellowing on exposure to sunlight, and the topcoat. Thus, the second primer layer also helps to block low molecular weight, light sensitive species from passing from the electrocoat layer into the topcoat layers where such species could cause a shift in color (yellowing) or other problems.
December et al., U.S. Pat. No. 5,431,791, disclose an electrodeposition coating composition comprising an aqueous mixture of a resin with a plurality of acid-salted primary amines groups and a curing agent with a plurality of cyclic carbonate groups. The composition is used to electrodeposit a coating layer onto an electrically conducting substrate. The deposited layer is cured to produce a cured coating layer. Electrocoat primer is usually overcoated with a primer-surfacer and at least one topcoat layer.
A primer composition has now been discovered that comprises a carbamate-functional novolac resin and an aminoplast crosslinker such as a melamine resin crosslinker. The primer composition is used to prepare a composite coating on a substrate, wherein the composite coating comprises at least one primer layer, wherein the outermost primer layer is obtained from applying and reacting the primer composition of the invention, and at least one topcoat layer that is applied onto the outermost primer coating layer.